The process of telemetry involves the transmission of data from a source to a receiver typically by radio frequency electromagnetic means. Telemetry has been successfully used in biomedical applications to permit the remote monitoring of patients. An example of a useful signal to be remotely monitored is the electrocardiogram ("ECG"). A transmitter attached to a patient sends an ECG signal to a remote receiver where it is decoded and monitored. Such biomedical telemetry permits the patient greater mobility than if he were attached to a stationary monitor, thus enhancing the therapeutic benefits from patient mobility.
Transmitters for biomedical telemetry typically use a double frequency modulation ("FM") scheme for encoding and transmitting the signal. The signal is encoded as a frequency modulation of a subcarrier, which is in turn modulated on a carrier frequency. In the case of an ECG signal, the subcarrier is typically deviated at +-20 Hz to +-300 Hz, the amount of frequency deviation being proportional to the ECG signal at the input to the transmitter. The typical subcarrier frequency is from 1 to 10 kHz, and the typical carrier frequency is in the range of either 174 to 216 Mhz or 450 to 470 Mhz. Ordinarily, the transmitter is designed by the manufacturer to operate at a set subcarrier and carrier frequency.
In typical telemetry applications, a receiver is employed which demodulates the carrier first. The subcarrier detection is then done with an analog circuit tuned to a particular transmitter characteristic. Such analog circuits are not designed to be changed by the user to permit their use with subcarrier frequencies for other manufacturers transmitters. Accordingly, such dedicated analog receiver circuits are not usable for decoding the subcarrier frequencies of a variety of manufacturers. Thus, if numerous transmitters with different subcarrier frequencies are being used, more than one receiver must be used.
One example of such a prior art biomedical telemetry demodulator is the Medical Data Electronic, Inc. ESCORT MODEL 1000 PORTABLE TELEMETRY STATION. Analog subcarrier detection circuitry is used which is set by the manufacturer to detect a fixed number of subcarrier frequencies used by one transmitter manufacturer.
Considering now the use of analog decode circuitry versus digital decode circuitry, a known disadvantage of using a digital circuit for purposes of radio frequency reception and demodulation is that the clock for the microprocessor can interfere with the desired signal. This has the undesired effect of reducing the signal to noise ratio for the desired signal.